asix: fix wrong return value in asix_check_host_enable()

If asix_read_cmd() returns 0 on 30th interation, 0 will be returned from
asix_check_host_enable(), which is logically wrong. Fix it by returning
-ETIMEDOUT explicitly if we have exceeded 30 iterations

Also, replaced 30 with #define as suggested by Andrew

Fixes: a786e319

 ("net: asix: fix uninit value bugs")
Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/ecd3470ce6c2d5697ac635d0d3b14a47defb4acb.1640117288.git.paskripkin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>